As proportionally more people with remaining life expectancies longer than about 10 years become recipients of total hip and knee implant prostheses, long-term mechanical stability of these devices becomes increasingly important. Cements used to hold the intermedullary shafts in place, such as methylmethacrylate, tend to loose with time, and the use of screws or simple press-fitting often leads to implant failure over time. A variety of alternatives to these common fixation methods are being tried. These include: (1) coarse textured surfaces, (2) welded or sintered onlays of metal beads or wires, (3) surfaces with three-dimensional interconnected porosity, and (4) ceramic coatings such as calcium phosphates or alumina. Efforts are also underway to develop permanent implantable teeth and other prosthetic devices that also require fixation to bone.
Grooves and other surface relief features achieve fixation by bone growth into them thus providing a mechanical stabilization of the device without necessarily achieving a true bone-to-prosthesis bond.
Sintered porous onlays such as beads or crimped wire are meant to allow bone tissue ingrowth thus forming a better bonding action than simple grooves, etc. Bone tissue can ingrow porous metal surfaces but the ingrown bone tissue has a strong tendency to necrose and resorb in a period of many months or a few years. Evidently, the mechanical stresses necessary for maintaining healthy bone are not adequately transmitted into the rigid metal porosity. Another problem has to do with crevice corrosion occurring at any weld surface. Even slight chemical difference due to flux, etc., at a weld can promote galvanic corrosion when placed in contact with the body's electrolytes over a long time.
Both the textured surface approach and the sintered porous onlays present major problems in the event a prostheses has to be removed.
Prostheses in which the intermedullary shaft surfaces are metallic with three-dimensional interconnected porosity, have been achieved by the casting of metal into sintered coral sleeves. The sintered coral (composed of calcium oxide, CaO) is subsequently removed with etching by a mild acid, leaving a complementary negative of the original porous structure of the sintered coral. Dunn et al. [Johns Hopkins Med. J. 145, 101 (1979)] have shown that prostheses prepared in this manner achieved some success when implanted into mongrel dogs, resulting in firm attachment by bony ingrowth. These prostheses, however, faced the same problems noted above for textured surfaces and porous onlays, i.e., showing some evidence of necrosis/resorbtion of ingrown bone tissue.
Ceramic coatings on metal prostheses offer promise because they may permit true bone-to-prosthesis bonding but have to maintain ceramic/metal interface integrity for many years.